Magnetic video disk recorder systems, which were first developed in the early 1960s, incorporate either rigid disk media for fixed but erasable storage or floppy disk media for removable storage. The disks typically rotate at video frame rate, 1500 RPM or 1800 RPM, and use saturated narrowband FM recording of the modulated analog signal with one complete disk revolution storing a video frame signal. A synchronized stepping motor is used for positioning the recording head over different tracks as video frames are recorded or replayed. Analog video disk recorders which are found in broadcast television, medical, and government applications achieve excellent specifications for video image storage density, cost per unit image capacity, acquisition and display bandwidth (real-time), and image access rates. Unfortunately, due to the short recording wavelength and high signal-to-noise ratio requirements of FM video, it becomes necessary in these systems to maintain head to media velocities of 1000 to 2000 inches per second, and head gap to media separations of only a few micro-inches. Floppy disk recorders require constant contact between head and coated oxide media in order to achieve adequate signal performance, thereby limiting useful head and media life to only a few hundred hours. Rigid disk recorders, on the other hand, greatly reduce contact wearing mechanisms by generating a moving film of air, or air bearing, to support the head at a fixed average distance above the media. The head flies 9 to 12 micro-inches above the surface in a high performance recorder. Depending on multiple factors including disk surface precision, air layer non-linearities near the mean free air molecular distance, head aerodynamic behavior, and head shock loading transients, contact between head and media occurs frequently enough to limit maintenance free periods in rigid disk analog recorders to about 2000 hours or less.
Relative to digital storage techniques, analog video recorders have fundamental limitations in signal fidelity, linearity, distortion artifacts, signal level repeatability, and time base accuracy. Indeed we are finding that newer digital radiography performance demands, digital processing compatability needs, and maintenance free reliability requirements are exceeding the present capabilities of analog video disk recorders. Consequently, video imaging system designers are turning more toward solid state computer memories and computer disk drives for high performance digital video image storage.
Modern fixed disk computer drives presently obtain maintenance-free operating periods of 8000 to 12,000 hours average. Computer disk systems, together with external disk controllers, are configured for fast random access of individual computer records, or data blocks, which are transferred to the controller buffer under control of the unsynchronized disk clock. However, due to the moderately slow transfer rates involved, only one data read/write channel is utilized with a large number of switched heads in the disk pack. The configuration of standard computer disk drives for fast access wideband video storage is at best a great mismatch in application which is inefficient and expensive. The digital video disk recorder of this invention uses established Winchester computer disk drive technology and video disk technology. Emphasizing high image quality performance and high reliability, the video disk recorder of this design enables synchronized real-time image data access, maximizes data transfer rate through high density parallel recording on relatively wide tracks, and eliminates the need for data controllers. The video disk recorder of this invention has an erasable storage capacity equivalent to a state-of-the-art 500 megabyte mainframe computer disk drive, but is a selfcontained recording system implemented at a practical level of size, complexity, and cost relative to digital instrumentation recorders or computer-based data acquisition systems.